Repeated administration of phytocannabinoid Δ(9)-THC or synthetic cannabinoids JWH-018 and JWH-073 induces tolerance to hypothermia but not locomotor suppression in mice, and reduces CB1 receptor expression and function in a brain region-specific manner.

These studies probed the relationship between intrinsic efficacy and tolerance/cross-tolerance between ∆(9)-THC and synthetic cannabinoid drugs of abuse (SCBs) by examining in vivo effects and cellular changes concomitant with their repeated administration in mice. Dose-effect relationships for hypothermic effects were determined in order to confirm that SCBs JWH-018 and JWH-073 are higher efficacy agonists than ∆(9)-THC in mice. Separate groups of mice were treated with saline, sub-maximal hypothermic doses of JWH-018 or JWH-073 (3.0mg/kg or 10.0mg/kg, respectively) or a maximally hypothermic dose of 30.0mg/kg ∆(9)-THC once per day for 5 consecutive days while core temperature and locomotor activity were monitored via biotelemetry. Repeated administration of all drugs resulted in tolerance to hypothermic effects, but not locomotor effects, and this tolerance was still evident 14 days after the last drug administration. Further studies treated mice with 30.0mg/kg ∆(9)-THC once per day for 4 days, then tested with SCBs on day 5. Mice with a ∆(9)-THC history were cross-tolerant to both SCBs, and this cross-tolerance also persisted 14 days after testing. Select brain regions from chronically treated mice were examined for changes in CB1 receptor expression and function. Expression and function of hypothalamic CB1Rs were reduced in mice receiving chronic drugs, but cortical CB1R expression and function were not altered. Collectively, these data demonstrate that repeated ∆(9)-THC, JWH-018 and JWH-073 can induce long-lasting tolerance to some in vivo effects, which is likely mediated by region-specific downregulation and desensitization of CB1Rs.

Cannabinoid CB1 receptor-mediated inhibition of prolactin release and signaling mechanisms in GH4C1 cells.

The GH4C1 cell line was used to study the cellular mechanisms of cannabinoid-mediated inhibition of PRL release. Cannabinoid CB1 receptor activation inhibited vasoactive intestinal polypeptide- and TRH-stimulated PRL release, but not its basal secretion. The cannabinoid-mediated inhibition of TRH-stimulated PRL release was reversed by the CB1 receptor-specific antagonist, SR141,716A, and was abolished by pertussis toxin pretreatment, indicating that G alpha subunits belonging to the G(i)alpha and G(o)alpha family were involved in the signaling. Photoaffinity labeling using [alpha-32P] azidoaniline GTP showed that cannabinoid receptor stimulation in cell membranes produced activation of four G alpha subunits (G(i)alpha2, G(i)alpha3, G(o)alpha1, and G(o)alpha2), which was also reversed by SR141,716A. The CB1 receptor agonists, WIN55,212-2 and CP55,940, inhibited cAMP formation and calcium currents in GH4C1 cells. The subtypes of calcium currents inhibited by WIN55,212-2 were characterized using holding potential sensitivity and calcium channel blockers. WIN55,212-2 inhibited the omega-conotoxin GVIA (Conus geographus)- and omega-agatoxin IVA (Aigelenopsis aperta)-sensitive calcium currents, but not the nisoldipine-sensitive calcium currents, suggesting the inhibition of N- and P-type, but not L-type, calcium currents. Taken together, the present findings indicate that CB1 receptors can couple through pertussis toxin-sensitive G alpha subunits to inhibit adenylyl cyclase and calcium currents and suppress PRL release from GH4C1 cells.

Protracted withdrawal: sensitization of the anxiogenic response to cocaine in rats concurrently treated with ethanol.

Rats were trained to respond on one lever following an injection of saline and the alternate lever after the anxiogenic drug pentylenetetrazol (PTZ 20 mg/kg), according to a fixed ratio (FR10) schedule of food reinforcement. The trained animals were then administered dependence-producing regimens of either cocaine (20 mg/kg, [IP], three times daily for 7 days) or ethanol (mixed 4.5% w/v with sweetened liquid diet given for 5 days). Separate groups of trained rats were given either subthreshold regimens of cocaine (20 mg/kg, IP, three times daily for 5 days), ethanol (2.25% w/v of the diet given for 5 days), or both. Additional groups were matched for control groups. After discontinuation of these regimens, rats were administered test injections of either saline or cocaine, and tested for elicitation of the PTZ-stimulus at selected intervals of withdrawal. After a saline injection, maximum elicitation of the PTZ-stimulus was observed 12 hours following chronic treatment with the higher dose of ethanol, and 120 hours following longer treatment with cocaine. During those periods of withdrawal when a saline injection failed to produce a PTZ-like stimulus, a test injection of cocaine (10 mg/kg) elicited the PTZ-stimulus in the ethanol withdrawn rats, although only partially eliciting the PTZ-stimulus in the cocaine withdrawn group. In the pair-fed controls, or rats withdrawn from the smaller dosage of either ethanol or cocaine, the test dose of saline or cocaine did not elicit the PTZ-stimulus; only 30% of rats selected the PTZ-appropriate level at the highest dose of cocaine tested (10 mg/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

Learning and memory-enhancing effects of Ro 15-4513: a comparison with flumazenil.

Synthetic benzodiazepines produce an anterograde amnesia, which can be reversed by selective benzodiazepine antagonists or inverse agonists. It has therefore been suggested that the memory-enhancing effects of the antagonists are due to antagonism of an endogenous "benzodiazepine-like" endocoid. If the memory-enhancing effects of the benzodiazepine antagonists are determined predominantly by the antagonism of such endogenous benzodiazepine-ligands, then it could be hypothesized that administration of an inverse agonist, which produces effects functionally opposite to those of benzodiazepine agonists, may also mimic the effects of benzodiazepine antagonists but not produce effects greater than those of the pure antagonists. The purpose of the present study was therefore to investigate the memory-enhancing effects of the benzodiazepine inverse agonist, ethyl-8-amido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo [1,5a] [1,4] benzodiazepine-3-carboxylate (Ro 15-4513) in young HSD:(ICR)BR mice and to compare these effects with those of the benzodiazepine antagonist, flumazenil. Pretraining injections of flumazenil and Ro 15-4513 (2.5 and 10.0 mg/kg) enhanced equally, both the acquisition and the retention of a task for 1 week requiring mice to discriminate the correct arm of a T-maze, to avoid a mild electric shock. Pretreatment with Ro 15-4513 also dose-dependently protected the animals from experimental amnesia, induced by the cholinergic receptor antagonist, scopolamine in a second model of memory, in which mice were required to passively avoid a dark chamber after shock. In contrast, Ro 15-4513, injected prior to daily active avoidance sessions, failed to significantly improve either the acquisition or retention performance.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of a kappa-opioid receptor expressed in CHO cells: interaction with multiple G-proteins is not specific for any individual G alpha subunit and is similar to that of other opioid receptors.

The purpose of the present study was to examine the coupling pattern of a recently cloned kappa-opioid receptor stably transfected in CHO cells to individual G alpha subunits with subsequent comparison to that observed previously for delta- and mu-opioid receptors. Data presented in the current study indicate the successful stable expression of a kappa-opioid receptor in CHO cells. This is supported by experiments in which ligands with selectivity for kappa-, but not delta- or mu-opioid receptors demonstrated high affinity for the expressed receptor and were able to potently and efficaciously produce inhibition of adenylyl cyclase activity. In addition, only kappa-opioid agonists were able to induce dose-dependent increases in the incorporation of [32P]azidoanilido-GTP into four G alpha subunits, three of which were identified as Gi3 alpha, Gi2 alpha and Go2 alpha. Further, the amount of kappa-opioid agonists required to induce 50% maximal labeling of any individual G alpha subunit was similar. Although kappa-opioid agonists produced equivalent maximal labelling of Gi3 alpha, Gi2 alpha and Go2 alpha, significantly less agonist-induced labeling was observed for an unknown G-protein designated as G? alpha. Although these results are slightly different than those observed previously for both delta- and mu-opioid receptors, it appears that all opioid receptors stably transfected in CHO cells interact with multiple G-proteins and that this coupling is not selective for any individual G alpha subunit.

Modulation of benzodiazepine agonist and inverse-agonist receptor binding by GABA during ethanol withdrawal.

1. The present study examined the capacity of GABA to modulate flunitrazepam and Ro15-4513 binding to putative GABAA receptors. Binding was measured in distinct brain regions both before and during selected periods of withdrawal from ethanol. 2. Rats were fed a nutritionally complete liquid ethanol (4.5% w/v) diet for 4 days and at various times after the last dose of ethanol (0, 12, 24, & 72 hr), rats were sacrificed and extensively washed brain membrane fractions were prepared. 3. Competitive inhibition of 3H-flunitrazepam binding by either flunitrazepam or Ro15-4513 (10(-10)M to 10(-7)M) was performed in the absence and presence of GABA (10(-5)M). In the presence of GABA, the apparent affinity for flunitrazepam was increased approximately 1.7 fold and the apparent affinity for Ro15-4513 was decreased by 1.7 fold. 4. No alteration in the capacity of GABA to modulate flunitrazepam or Ro15-4513 affinity (e.g. GABA-shift) was observed in cortical membrane preparations either 12 or 72 hr following ethanol cessation. 5. Further, no changes in GABA-modulation of flunitrazepam binding was evident 0, 12, 24, or 72 hr after the last ethanol dose in membranes prepared from cortex, hippocampus or cerebellum. 6. Therefore, results from the present study indicate that the capacity of GABA to modulate receptor affinity for benzodiazepine agonists and inverse-agonists in rat cortex, hippocampus or cerebellum is not altered during withdrawal from chronic ethanol.

The benzodiazepine receptor inverse agonist RO 15-3505 reverses recent memory deficits in aged mice.

The benzodiazepine receptor partial inverse agonist RO 15-3505 was tested for its ability to improve impaired recent memory of aged mice. All mice successfully acquired a learning set for accurate identification of the correct arm of a T-maze and could perform with nearly 100% accuracy after 1-min delays. However, performance of the aged mice approached chance levels after 2-h delays. When injected just before testing on a series of 2-h retention tests, RO 15-3505 (from 2.5-3505 (from 2.5-10.0 mg/kg) resulted in a marked improvement of response accuracy. These results confirm the role of benzodiazepine receptor mechanisms in the modulation of memory processes, and suggest that the memory-facilitating effects RO 15-3505 or similar benzodiazepine receptor ligands may be generalized to aged rodents with impaired memory function.

Anxiogenic behavior in rats during acute and protracted ethanol withdrawal: reversal by buspirone.

This study investigated the effectiveness of buspirone in reversing the anxiogenic behaviors occurring during ethanol withdrawal as measured in the elevated plus-maze. In response to anxiogenic drugs, rats spend less time in and make fewer entries onto the open arms of an elevated plus-maze, whereas anxiolytic drugs produce opposite effects. In this study, rats were fed a liquid diet containing 4.5% ethanol for 7 days. Twelve h (acute withdrawal) and 7 days (protracted withdrawal) following cessation of the ethanol diet, rats were tested on the elevated plus-maze. During these withdrawal periods, the percent open-arm entries and time spent on the open arms were significantly reduced relative to animals fed an ethanol-free diet, suggestive of anxiogenic-like symptoms. Buspirone (0.32-1.25 mg/kg) dose dependently reversed the withdrawal-induced decreases in open-arm activity. The anxiolytic-like activity of buspirone observed during ethanol withdrawal may be due to a reduction in serotonergic neurotransmission through activation of presynaptic 5-HT1A autoreceptors. The results obtained in this study suggest that pharmacotherapy with selective 5-HT1A agonists may be beneficial in alleviation of anxiety during ethanol withdrawal.

Sensitization to 5-HT1C receptor agonist in rats observed following withdrawal from chronic ethanol.

Anxiogenic action of m-chlorophenylpiperazine (mCPP), a 5-HT1C receptor agonist, was studied in naive rats and in ethanol-tolerant rats following withdrawal from chronic ethanol administration. The purpose of this investigation was to determine whether a sensitization to mCPP develops during withdrawal from chronic ethanol. Male Long-Evans hooded rats were fed a liquid diet containing 4.5% ethanol or dextrin (as control) for four days. Twelve hours (acute withdrawal) or 4 days (protracted withdrawal) after the last dose of ethanol, rats were injected with saline or mCPP (0.08-5.0 mg/kg) and were tested in the elevated plus-maze 15 min postinjection. A reduction in percent open-arm activity, indicative of anxiogenic behavior, was observed in ethanol-treated rats injected with saline. Administration of mCPP further reduced the percent open-arm entries and time in ethanol-withdrawn rats. An eightfold reduction in maximum effective dose of mCPP was observed during acute ethanol withdrawal as compared to that in naive rats. During protracted ethanol withdrawal the maximum effective dose of mCPP was reduced by 75%. A shift of the mCPP dose-response curve to the left following withdrawal from chronic ethanol may indicate that 5-HT1C receptor sites are more sensitive to the activation by an agonist. This effect may be exploited in developing specific 5-HT1C receptor antagonists for the treatment of ethanol withdrawal symptoms.

Anxiogenic effect of disulfiram evaluated in an animal model.

The present experiment was designed to determine whether disulfiram produced an interoceptive discriminative stimulus (IDS) in rats similar to that exhibited by the anxiogenic drug pentylenetetrazol (PTZ). Rats were trained to discriminate PTZ (20 mg/kg IP) from saline, according to an FR10 schedule of food reinforcement. After training criteria had been met, discrimination testing revealed that disulfiram (100 mg/kg, IP) produced a PTZ-like IDS that fully substituted for PTZ 4 hours after injection, decaying to baseline values by the third day. The metabolite of disulfiram, diethyl dithiocarbamate (100 mg/kg, IP), followed much the same temporal pattern, but only showed partial substitution for PTZ. When ethanol (1 g/kg, gavage) or the monoamine oxidase inhibitor pargyline (50 mg/kg, IP) were administered in combination with disulfiram, no significant change of the PTZ-like stimulus was observed. Pargyline (100 mg/kg, IP) or acetaldehyde (200 mg/kg, IP), given alone, did not significantly substitute for PTZ. These data show that disulfiram produces an IDS in rats similar to that produced by other anxiogenic drugs. These data also suggest that the mechanism by which disulfiram produces its anxiogenic effect may not be entirely based upon the disulfiram metabolite diethyl dithiocarbamate, elevated acetaldehyde levels or stimulation of the catecholaminergic system.

In vivo effects of synthetic cannabinoids JWH-018 and JWH-073 and phytocannabinoid Δ9-THC in mice: inhalation versus intraperitoneal injection.

Human users of synthetic cannabinoids (SCBs) JWH-018 and JWH-073 typically smoke these drugs, but preclinical studies usually rely on injection for drug delivery. We used the cannabinoid tetrad and drug discrimination to compare in vivo effects of inhaled drugs with injected doses of these two SCBs, as well as with the phytocannabinoid Δ(9)-tetrahydrocannabinol (Δ(9)-THC). Mice inhaled various doses of Δ(9)-THC, JWH-018 or JWH-073, or were injected intraperitoneally (IP) with these same compounds. Rectal temperature, tail flick latency in response to radiant heat, horizontal bar catalepsy, and suppression of locomotor activity were assessed in each animal. In separate studies, mice were trained to discriminate Δ(9)-THC (IP) from saline, and tests were performed with inhaled or injected doses of the SCBs. Both SCBs elicited Δ(9)-THC-like effects across both routes of administration, and effects following inhalation were attenuated by pretreatment with the CB1 antagonist/inverse agonist rimonabant. No cataleptic effects were observed following inhalation, but all compounds induced catalepsy following injection. Injected JWH-018 and JWH-073 fully substituted for Δ(9)-THC, but substitution was partial (JWH-073) or required relatively higher doses (JWH-018) when drugs were inhaled. These studies demonstrate that the SCBs JWH-018 and JWH-073 elicit dose-dependent, CB1 receptor-mediated Δ(9)-THC-like effects in mice when delivered via inhalation or via injection. Across these routes of administration, differences in cataleptic effects and, perhaps, discriminative stimulus effects, may implicate the involvement of active metabolites of these compounds.

Interaction of co-expressed mu- and delta-opioid receptors in transfected rat pituitary GH(3) cells.

mu- and delta-Opioid agonists interact in a synergistic manner to produce analgesia in several animal models. Additionally, receptor binding studies using membranes derived from brain tissue indicate that interactions between mu- and delta-opioid receptors might be responsible for the observation of multiple opioid receptor subtypes. To examine potential interactions between mu- and delta-opioid receptors, we examined receptor binding and functional characteristics of mu-, delta-, or both mu- and delta-opioid receptors stably transfected in rat pituitary GH(3) cells (GH(3)MOR, GH(3)DOR, and GH(3)MORDOR, respectively). Saturation and competition binding experiments revealed that coexpression of mu- and delta-opioid receptors resulted in the appearance of multiple affinity states for mu- but not delta-opioid receptors. Additionally, coadministration of selective mu- and delta-opioid agonists in GH(3)MORDOR cells resulted in a synergistic competition with [(3)H][D-Pen(2,5)]enkephalin (DPDPE) for delta-opioid receptors. Finally, when equally effective concentrations of [D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin (DAMGO) and two different delta-opioid agonists (DPDPE or 2-methyl-4a alpha-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12a alpha-octahydroquinolino-[2,3,3-g]-isoquinoline; TAN67) were coadministered in GH(3)MORDOR cells, a synergistic inhibition of adenylyl cyclase activity was observed. These results strongly suggest that cotransfection of mu- and delta-opioid receptors alters the binding and functional characteristics of the receptors. Therefore, we propose that the simultaneous exposure of GH(3)MORDOR cells to selective mu- and delta-opioid agonists produces an interaction between receptors resulting in enhanced receptor binding. This effect is translated into an augmented ability of these agonists to inhibit adenylyl cyclase activity. Similar interactions occurring in neurons that express both mu- and delta-opioid receptors could explain observations of multiple opioid receptor subtypes in receptor binding studies and the synergistic interaction of mu- and delta-opioids in analgesic assays.

Chronic exposure to mu-opioid agonists produces constitutive activation of mu-opioid receptors in direct proportion to the efficacy of the agonist used for pretreatment.

Chronic morphine treatment has been shown to produce constitutive activation of mu-opioid receptors, and this transition might contribute to the development of tolerance and dependence. The apparent ability of chronic morphine to increase the spontaneous, agonist-independent activation of mu-opioid receptors may be unique, due to its distinct partial agonist properties of possessing a relatively high intrinsic activity coupled with a poor ability to produce desensitization and down-regulation. Therefore, the present study tested the hypothesis that prolonged exposure to morphine would produce greater constitutive activity of mu-opioid receptors than exposure to the full agonist [D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin (DAMGO). GH(3) cells expressing mu-opioid receptors were exposed to chronic morphine, DAMGO, or no opioid under conditions determined to produce maximal desensitization, down-regulation, and cAMP rebound. After chronic treatment, the mu-opioid antagonists naloxone and beta-chlornaltrexamine (beta-CNA) were evaluated in two assays predictive of inverse agonist activity. Both antagonists produced a concentration-dependent inhibition of [(35)S]GTP gamma S binding only in membranes prepared from cells chronically exposed to opioids. This effect was reversed by the neutral mu-opioid antagonist CTAP. Additionally, conditions known to uncouple G protein-coupled receptors from G proteins produced a leftward shift in the competition curve of beta-CNA for [(3)H]DAMGO binding only in membranes prepared from chronically treated cells. In contrast, these conditions produced no shift in the competition curve by the neutral antagonist CTAP in cells exposed to chronic DAMGO. Therefore, prolonged exposure of GH(3)MOR cells to opioids produced constitutive activation of mu-opioid receptors. Surprisingly, chronic treatment with the more efficacious agonist DAMGO produced greater increases in both measures of inverse agonist activity than did morphine. These observations may lend novel insight into the mechanisms of opioid tolerance and dependence.

Mianserin in the treatment of ethanol withdrawal in the rat: prevention of behaviors indicative of anxiety.

The present investigation was a pilot study to determine whether a single dose of mianserin, which produces long-term down-regulation of serotonin1C (5-HT1c) and 5-HT2 receptors, would prevent anxiogenic behaviors occurring during ethanol withdrawal as evaluated in the elevated plus maze. Male Long-Evans hooded rats were fed a liquid diet containing 4.5 percent ethanol for 4 days. Mianserin (20 mg/kg, i.p.) was injected on the morning of the third day of ethanol administration, or 48 hrs and 7 days prior to testing. When animals were tested either 12 hrs (acute withdrawal) or 5 days (protracted withdrawal) after the last dose of ethanol, anxiogenic behaviors were observed as a significant reduction in both percentage of open-arm entries and time spent on the open arms. In contrast, these anxiogenic behaviors were prevented by pre-injection with mianserin 48 hrs or 7 days prior to testing. Attenuation of this important symptom of ethanol withdrawal is of particular importance because, in addition to the nonaddicting properties of mianserin relative to current anxiolytics, the beneficial effects appear to be long lasting and can be achieved with a single dose.

Mu and delta opioid receptor desensitization in undifferentiated human neuroblastoma SHSY5Y cells.

Both mu and delta opioid receptors are expressed in undifferentiated human neuroblastoma SHSY5Y cells and are negatively coupled to adenylate cyclase. The ability of various mu opioid, delta opioid and alpha-2 adrenergic agonists to inhibit acutely forskolin-stimulated adenylate cyclase activity in undifferentiated SHSY5Y cells after chronic administration with the selective mu opioid agonist [N-MePhe3,D-Pro4]morphiceptin (PLO17) or delta opioid agonist, [D-Pen2,D-Pen5]enkephalin (DPDPE) was assessed. In control cells, both PLO17 and DPDPE inhibited cyclic AMP (cAMP) formation with equal maximal inhibition, i.e., 60 +/- 3 and 66 +/- 2%, having IC50 values of 51.1 +/- 1.3 and 3.7 +/- 1.0 nM, respectively. The inhibition of intracellular cAMP formation by both agonists could be blocked by pertussis toxin pretreatment. After 24 hr of chronic administration of PLO17 (50 nM to 10 microM), a concentration-dependent loss of the ability of mu opioid agonists PLO17 and DAMGO, but not the delta opioid agonists DPDPE, nor alpha-2 adrenergic agonist UK-14304 (5-Bromo-N-(4,5,-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine) to inhibit adenylate cyclase activity was observed. In contrast, chronic administration of DPDPE (0.1 nM to 0.3 microM) resulted in a concentration-dependent reduction in the inhibition of cAMP formation produced by delta opioid agonists DPDPE and DSLET, but not mu opioid, nor alpha-2 adrenergic agonists tested. The observed homologous desensitization was also time-dependent. In addition, antagonist-induced increases in adenylate cyclase activity were observed only after chronic PLO17 administration.2+ Finally, chronic pretreatment of cells with PLO17 (10 microM) resulted in a significant decrease in mu opioid, but not delta opioid receptor, binding, whereas treatment with DPDPE (0.3 microM) resulted in a significant decrease in delta opioid, but not mu opioid receptor binding. Therefore, undifferentiated SHSY5Y cells may provide an excellent model system to study not only the signal transduction mechanisms of mu and/or delta opioid receptors, but also the cellular adaptations of specific opioid receptors.

Constitutively active mu-opioid receptors inhibit adenylyl cyclase activity in intact cells and activate G-proteins differently than the agonist [D-Ala2,N-MePhe4,Gly-ol5]enkephalin.

The most convincing evidence demonstrating constitutive activation of mu-opioid receptors is the observation that putative inverse agonists decrease basal G-protein activity in membrane preparations. However, it is not clear whether constitutively active receptors in isolated membranes have any physiological relevance in intact cells. GH3 cells expressing mu-opioid receptors (GH3MOR) exhibit higher basal G-protein activity and lower basal cAMP levels than wild-type GH3 cells, indicative of constitutively active receptors. This study determined whether alkylation of mu-opioid receptors by the irreversible antagonist beta-funaltrexamine would decrease spontaneous receptor activity in intact cells, revealing constitutive activity. GH3MOR cells were pretreated with increasing concentrations of beta-funaltrexamine followed by functional testing after removal of unbound drug. beta-Funaltrexamine pretreatment produced a concentration-dependent decrease in mu-opioid receptor binding with an IC50 of 0.98 nm and an Emax of 77%. Similar concentrations of beta-funaltrexamine pretreatment produced a half-maximal reduction in basal [35S]GTPgammaS binding, a decrease in basal photolabeling of G-proteins with azidoanilido-[alpha-32P]GTP, and an increase in basal adenylyl cyclase activity in intact cells. Therefore, mu-opioid receptors are constitutively active in intact cells, producing stimulation of G-proteins and inhibition of adenylyl cyclase. Importantly, photolabeling of Galpha-subunits with azidoanilido-[alpha-32P]GTP demonstrated that constitutively active mu-opioid receptors activate individual G-proteins differently than the agonist [d-Ala2,N-MePhe4,Gly-ol5]enkephalin.

Agonist Activity of the delta-antagonists TIPP and TIPP-psi in cellular models expressing endogenous or transfected delta-opioid receptors.

A new class of highly selective delta-opioid receptor antagonists has been recently developed, termed the TIP(P) peptides. Two prototypical compounds in this class are TIPP (H-Tyr-Tic-Phe-Phe-OH) and a derivative, TIPP-psi (H-Tyr-Tic[CH2NH]-Phe-Phe-OH). Surprisingly, both TIPP and TIPP-psi demonstrated inhibition of adenylyl cyclase activity in GH3 cells transfected with delta-opioid receptors (GH3DORT), an effect normally observed by agonists. The agonist activity was delta-selective, because no inhibition occurred in wild-type GH3 or GH3MOR (mu-opioid receptor) cells. Both TIPP and TIPP-psi exhibited concentration-dependent inhibition of adenylyl cyclase activity; however, TIPP-psi was found to be less potent (IC50 = 3.97 versus 0.162 nM) and less efficacious (I(max) = 50% versus 70%) than TIPP. Pretreatment of cells with pertussis toxin attenuated the inhibition of maximally effective concentrations of TIPP and TIPP-psi, indicating the involvement of G(i)alpha/G(o)alpha G-proteins. Other delta-antagonists, naltriben, naloxone, and ICI 174864, attenuated the inhibition of adenylyl cyclase activity mediated by TIPP. Coadministration of TIPP with the selective delta-agonist [D-Pen2,5]enkephalin resulted in an additive interaction. Both TIPP and TIPP-psi exhibited significant inhibition of adenylyl cyclase activity in different GH3DORT clones expressing a 28-fold range of delta-opioid receptor densities, and in cell lines expressing endogenous (i.e., N1E115 and NG108-15) and transfected (i.e., Chinese hamster ovary-DOR and human embryonic kidney-DOR) delta-opioid receptors, with densities ranging from 0.12 to 6.67 pmol/mg. These results suggest that compounds previously thought to be purely delta-opioid receptor antagonists also demonstrate agonist activity in several in vitro models.

Interaction of delta-opioid receptors with multiple G proteins: a non-relationship between agonist potency to inhibit adenylyl cyclase and to activate G proteins.

The purpose of the present investigation was to determine whether the coupling of delta-opioid receptors to multiple G proteins in NG108-15 neuroblastoma x glioma cells is a characteristic limited to only this cell line (because of the high density of delta-opioid receptors) and to ascertain whether there is any correlation between delta-opioid agonist potency to inhibit adenylyl cyclase and to activate G proteins. Interactions between receptors and G proteins were investigated using agonist-stimulated incorporation of the photoreactive GTP analog azidoanilido[alpha-32P]GTP ([alpha-32P]AA-GTP) into G protein alpha subunits, with subsequent separation by urea/sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In NG108-15, NS20Y, and N1E115 cell membranes, four alpha subunits (Gi2 alpha, one isoform of Gi3 alpha, and both isoforms of Go alpha) in the 39-41-kDa region were labeled with [alpha-32P]AA-GTP. The delta-opioid agonist [D-Ala2,D-Leu5]-enkephalin (DADLE) produced a dose-dependent, naloxone-reversible increase of [alpha-32P]AA-GTP incorporation into all four alpha subunit subtypes, in all cell lines tested. In addition, with the single exception of Gi3 alpha in NG108-15 cells, the maximal increases in incorporation of the photoaffinity label into all G alpha subunits induced by DADLE were similar. The Bmax values determined for delta-opioid receptors in NG108-15, NS20Y, and N1E115 cell membranes were 570, 370, and 120 fmol/mg of protein, respectively. Finally, although the IC50 values to inhibit intracellular cAMP production and affinity for DADLE were similar across the three cell lines, the EC50 values to produce labeling of the G alpha subunits between cell lines differed by > 100-fold. In fact, only in NS20Y cells were the IC50 and ED50 values comparable. Firstly, these results suggest that simultaneous coupling of the delta-opioid receptor to multiple G protein alpha subunits occurs in a variety of cell lines that express a range of receptor densities. Secondly, the magnitudes with which delta-opioid receptors interact with available G alpha subunits in response to agonist are approximately the same. Finally, there appears to be no relationship between the potency of agonists to inhibit adenylyl cyclase and that required for activation of G proteins.

Potential role of 5HT1C and/or 5HT2 receptors in the mianserin-induced prevention of anxiogenic behaviors occurring during ethanol withdrawal.

A single dose of mianserin (a 5HT1C/5HT2 antagonist), administered 1 hr, 48 hr, or 7 days before testing, was evaluated for its efficacy in alleviating or preventing the occurrence of anxiogenic behaviors observed during ethanol withdrawal. Other behavioral experiments using selected drug interactions were conducted to examine whether the effect of mianserin was related to a long-term modification of 5-hydroxy-tryptamine (5HT) receptor function. Rats were fed a liquid diet containing 4.5% ethanol for 4 days. They were tested on the elevated plus-maze (EPM) 12 hr (acute withdrawal) and 5 days (protracted withdrawal) after the last ethanol dose. Ethanol withdrawal induced a pattern of "anxiogenic" behavior that consisted of reduced activity (total entries) and a reduced proportion of open arm activity. Mianserin, injected as a single dose given either 1 hr (0.16-5 mg/kg, ip) before testing or given (20 mg/kg, ip) on the morning of the 3rd day of ethanol administration, i.e., 48 hr and 7 days before testing, dose-dependently prevented or reversed the ethanol withdrawal induced reduction in open-arm activity. In contrast, the 5HT1C/5HT2 receptor agonist (+/- )-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOl) did not affect behaviors in the EPM in ethanol-naive rats, nor in those undergoing ethanol withdrawal. However, although there was a marked tolerance to DOl-induced body shakes (a measure of 5HT2 function) during withdrawal, DOl reversed the action of mianserin in the EPM. The 5HT1 receptor agonist, 5HT2 receptor antagonist 1-naphthyl-piperazine (1-NP) reduced open-arm activity in ethanol-naive rats and this action was enhanced during withdrawal.(ABSTRACT TRUNCATED AT 250 WORDS)